Electrophotographic (EP) processes for producing a permanent image on media are well known and commonly used. In general, a common process includes: (1) charging a photoreceptor (optical photoconductor or OPC) such as a roller or continuous belt bearing a photoconductive material; (2) exposing the charged photoreceptor to imaging light (laser) that discharges the photoreceptor in select areas to define a latent electrostatic image on the photoreceptor; (3) presenting developer particles (toner) to the photoreceptor surface bearing the image so that the particles are transferred to the surface in the shape of the image; (4) transferring the particles in the shape of the image from the photoreceptor to the media; (5) fusing or fixing the particles in the shape of the image to the media; and (6) cleaning or restoring the photoreceptor for the next printing cycle. Many image forming apparatus, such as laser printers, copy machines, and facsimile machines, utilize this well known electrophotographic printing process.
Laser driven color printers and copiers employ toners that enable light to reflect off the page and to be directed back towards the eye. In general, such devices employ cyan (C), magenta (M) and yellow (Y) toners as the principal component colors, from which other colors are created. Light passing through CMY toners has part of its color filtered out or absorbed by the toner such that the reflected light takes on the color of the toners that it passes through. In laser printers (and some copiers), a black (K) toner is also used which is opaque to light. When a printer receives image data from a host processor, the data is received in the form of either Red, Green and Blue (RGB) values, CMYK values, L*a*b* or some other conventional color space values. In any case, the received values are typically converted to CMYK values in order to achieve desired levels of color representation on the final imaged document.
Many color EP devices, such as color laser printers, utilize a four-pass process to produce a full-color CMYK image on a photoconductor. For example, FIG. 1 is a block diagram depicting a conventional EP system wherein four developer modules 1,2, 3 and 4 are arranged along a moving photoconductor surface/drum 5. Each developer module is allocated to the deposition of one of the CMY and K toners onto the moving photoconductor 5. A charging station (corona) 6 uniformly charges the photoconductor 5 and an exposure station (laser light) 7 selectively discharges the photoconductor in accordance with a color plane's image data. The imaged photoconductor 5 then moves past the respective developer modules, with one developer module being moved into juxtaposition with the photoconductor (such as is shown with black developer 4) to allow color toning of the discharged areas. The developed photoconductor then experiences a full rotation, is charged again 6, and then exposed again in accordance with a next color plane's data and again developed, using the next color developer. The procedure continues until four passes have occurred and a full color image is present on the photoconductor 5. Thereafter, the image is electrostatically transferred via a transfer roller 9 to a sheet media 11 and subsequently fused to the sheet media 11 by fuser roller 12.
Alternatively, after development of any given color plane, that color plane's image on the photoconductor surface may be transferred to an intermediate transfer (IT) member (not shown) prior to ultimately being transferred to the sheet media. Once all color planes are transferred to the IT member, only then is the entire, full color image transferred to the media. This is commonly known as indirect transfer. To clarify, for example, upon each revolution of the photoconductor, one color plane will be imaged on the photoconductor and then immediately transferred to the IT member before a next color plane is similarly imaged and the process repeated. Once the IT member holds all of the color planes forming the final color image, the image is then transferred to the sheet media. In such indirect transfer, the IT member is generally large enough to hold an entire image plane at one time. Whether direct or indirect electrostatic image transfer occurs, the resultant image of toner is subsequently fused to the sheet media.
In EP systems, "hot offset" occurs when the fuser 12 picks up toner from a sheet 11 currently being fused and, depending upon fuser roller size and sheet size, transfers that picked up toner to a trailing portion 13 of the sheet or to a next sheet fused (not shown). The hot offset transfer is typically noticed as a "shadow" of the source image originally fused. Hot offset tends to occur more with certain black toners than with other non-black color toners such as cyan, magenta or yellow. Additionally, hot offset tends to occur and tends to be more noticeable when extremely smooth, less absorbing, or coated media are imaged, such as overhead transparencies (OHTs), partially because these media characteristics or coatings act as a smooth barrier that disallows the toner from absorbing more completely into the media. For ease of discussion purposes, any media that have characteristics of an extremely smooth surface, or are generally less absorbing, or are coated to achieve such a surface condition (all relative to conventional paper media typically used in printers and copies), will be referred to herein as "coated" media.
FIG. 2 depicts an example of a conventional black text image 14 fused onto a coated sheet media 11, and further depicts a hot offset "shadow" occurrence 15 of the original text image 14 disposed near the trailing edge of the sheet. The black text image 14 is fused onto the sheet 11 by the fuser 12 but, undesirably, some of the toner particles from the image 14 are retained on the fuser. Consequently, as the trailing end 13 of the sheet 11 passes through the fuser 12, the shadow image 15 is formed.
Accordingly, an object of the present invention is to reduce or eliminate hot offset in a color imaging device.